In some applications, it is desirable to expose an internal portion of an electric motor to fluid under high pressure. For example, in the field of shock absorbers, it may be desirable to provide a hydraulic actuator having an electric motor capable of withstanding high fluid pressures up to 1500 PSI (10,342 kPa) at the rotor without leakage of hydraulic fluid into the stator, where the fluid would damage the stator windings, cause the motor to malfunction, and degrade actuator performance by reducing the amount of hydraulic fluid available for operating the actuator.
It is known to protect the stator of an electric motor from fluid damage by encasing the stator in overmolded resin. While this approach is generally effective for applications in which the fluid is at relatively low pressures, it is not suitable for applications involving higher fluid pressures because the porosity of the molded resin cannot be entirely eliminated and leakage may occur. Also, adhesion between the molded resin and an external housing of the motor will degrade over time when the motor is subjected to wide ranges of temperature cycling and pressure cycling. While it is theoretically possible to seal a stator from intrusion of high pressure fluid using only overmolded resin, doing so for high volume production of electric motors is highly impractical and would be very expensive because a porosity-free surface finish and tight dimensional tolerances of the resin must be ensured.
There is a need for an electric motor capable of withstanding high fluid pressures, i.e. pressures up to about 1500 PSI (10,342 kPa), which can be reliably mass-produced at a reasonable cost point.